They exist because of the economic context at the time and place that they were generated, which may not have been valid for much more than that immediate context.
If you can find and examine the mathematical assumptions about cost and risk that justify the rule of thumb against current conditions, then you can make an informed evaluation of whether or not it applies to the current situation. If you can’t, like folk remedies, the advice is as likely to harmful as helpful in your actual concrete circumstances.
